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Spinosaurids Temporal range: Late Jurassic–Late Cretaceous, Possible Late Maastrichtian record
Skeletal reconstruction of Spinosaurus aegyptiacus
Scientific classification
Missing taxonomy template (fix):	PaleoGeekSquared/sandbox
Type species
Spinosaurus aegyptiacus Stromer, 1915
Subgroups
†"Sinopliosaurus" fusuiensis †Siamosaurus †Ostafrikasaurus †Baryonyx †Cristatusaurus †Ichthyovenator †Suchomimus †Suchosaurus †Spinosaurinae †Irritator †Oxalaia †Spinosaurus †Sigilmassasaurus
Synonyms
Baryonychidae Charig & Milner, 1986 Irritatoridae Martill et al., 1996 Sigilmassasauridae Russel, 1996

Spinosauridae (meaning 'spined lizards') is a family of megalosauroidean theropod dinosaurs, it contains the eponymous Spinosaurus, which is currently one of if not the largest terrestrial predator known from the fossil record, and might have measured up to 15 m (49 ft) in length. Most spinosaurids lived during the Cretaceous period, and fossils of them have been recovered worldwide, including Africa, Europe, South America, Asia and Australia.

They were large bipedal carnivores with elongated, crocodile-like skulls, sporting conical teeth with no or only very tiny serrations, and oftentimes small crests on top of their heads. The teeth in the front end of the lower jaw fanned out into a structure called a rosette, which gave the animal a characteristic look. Their shoulder blades were robust and hook-shaped, bearing relatively large forelimbs with enlarged claws on the first digit of their hands. Some genera also had unusually elongated neural spines, which might have supported sails or humps made of skin and fat tissue.

Sediment analysis has revealed that most spinosaurids inhabited fresh-to-brackish shallow water habitats in tropical regions, often near lagoons, marshes, and mudflats. These environments would have been optimal for a semi-aquatic lifestyle, which has also been corroborated by isotope and osteological (bone) analyses done on several taxa in this clade. Coupled with their conical teeth, designed for gripping prey instead of ripping flesh, this would have made them specialists, hunting mostly fish, while feeding opportunistically on other animals.

Description

Scale diagram of a few spinosaurids, Spinosaurus, Suchomimus, Baryonyx, and Irritator

Anatomical characteristics

Spinosaurids have relatively large forearms and an enlarged claw on the first digit of the hand. They have a hook shaped coracoid, external nares which are at least behind the teeth of the premaxillae or even further posterior on the skull, a long secondary palate, a terminal rosette of enlarged teeth at the front of both the upper and lower jaws, and subconical teeth with small denticles or no denticles.

Dorsal sails

Spinosaurus aegyptiacus, the type species for the family and subfamily, is known for the vertebrae with elongated neural spines, some over a meter tall, which have been reconstructed as a sail or hump running down its back.

Variation of neural spines in 3 different spinosaurid species

In Ichthyovenator, this sail is a half a meter at its highest and split into two at the sacral vertebrae. Suchomimus also has a low, ridge-like sail over its hips, smaller than that of Spinosaurus. Baryonyx, however, lacks a sail. Because of phylogenetic bracketing, other spinosaurids such as Irritator might have also had extended neural spines, but sufficient fossil vertebra are needed to know for certain.

Bony crests

Another feature that makes spinosaurid skulls unique from most other theropods, is that they often feature relatively large sagittal crests formed from their nasal bones. These crests have been present in Spinosaurus as a ridge-shaped structure, and in Suchomimus and Baryonyx as smaller bumps on top of the skull. They have also been seen (to a smaller degree) in , and Cristatusaurus.

Evolutionary history

Timespan

Suchomimus and Baryonyx to scale

The spinosaurids are known to exist from as early as the Late Jurassic, through characteristic teeth which were found in Tendaguru, Tanzania, and attributed to Ostafrikasaurus, 15 Million years prior to Siamosaurus. Baryonychines were common, as represented by Baryonyx, which lived during the Barremian of England and Spain. Baryonyx-like teeth are found from the earlier Hauterivian and later Aptian sediments of Spain, as well as the Hauterivian of England, and the Aptian of Niger. The earliest record of spinosaurines is from Africa; they are present in Albian sediments of Tunisia and Algeria, and in Cenomanian sediments of Egypt and Morocco. Spinosaurines are also found in Hauterivian and Aptian-Albian sediments of Thailand, and Southern China. In Africa, baronychines were common in the Aptian, and then replaced by spinosaurines in the Albian and Cenomanian.

Some intermediate specimens extend the known range of spinosaurids past the youngest dates of named taxa. A single baryonychine tooth was found from the mid-Santonian, in the Majiacun Formation of Henan, China. Possible spinosaur remains were also reported from the late Maastrichtian Maevarano Formation.

Localities

Map illustrating spinosaurid fossil discoveries

Confirmed spinosaurids have been found on every continent except for North America and Antarctica. The first of which was discovered in 1912 at the Bahariya Formation in Egypt and described in 1915 as Spinosaurus aegyptiacus. Over the years Africa has shown a great abundance in spinosaurid discoveries, such as in the Kem Kem beds of Morocco, which housed an ecosystem full of many large coexisting predators. A fragment of a spinosaurine lower jaw from the Early Cretaceous was also reported from Tunisia, and referred to Spinosaurus.

Various views of the Australian Spinosaurid specimen, one on bottom right is of Baryonyx for comparison.

Spinosaurinae's range has also extended to South America, particularly Brazil, with the discoveries of Irritator, Angaturama, and Oxalaia. There was also a fossil tooth in Argentina which has been referred to spinosauridae by Salgado et al. This referral is doubted by Tanaka, who offers Hamadasuchus, a crocodilian, as the most likely animal of origin for these teeth.

Baryonychines have been found in Africa, with Suchomimus and Cristatusaurus, as well as in Europe, with Baryonyx and Suchosaurus. Baryonyx-like teeth are also reported from the Ashdown Sands of Sussex, in England, and the Burgos Province, in Spain. A partial skeleton and many fossil teeth also hint at the possibility of spinosaurids being widespread in Asia. As of 2012, three have been named: Ichthyovenator; a baryonychine, and Siamosaurus and "Sinopliosaurus" fusuiensis; two indeterminate spinosaurids. At la Cantalera-1, a site in the Early Barremanian Blesa Formation in Treul, Spain, two types of spinosaurid teeth were found, and they were assigned, tentatively, as indeterminate spinosaurine and baryonychine taxa.

An intermediate spinosaurid was discovered in the Early Cretaceous Eumeralla Formation, Australia. It is known from a single 4 cm long partial cervical vertebra, designated P221081. It is missing most of the neural arch. The specimen is from a juvenile estimated to be about 2 to 3 meters long (6-9 ft). Out of all spinosaurs it most closely resembles Baryonyx.

Classification

Life restoration of Irritator

The family Spinosauridae was named by Ernst Stromer in 1915 to include the single genus Spinosaurus. The clade was expanded as more close relatives of Spinosaurus were uncovered. The first cladistic definition of spinosauridae was provided by Paul Sereno in 1998 (as "All spinosaurids closer to Spinosaurus than to Torvosaurus).

Taxonomy

Traditionally, spinosauridae is divided into two subfamilies: spinosaurinae, which contains the genera Irritator, Oxalaia, and Spinosaurus, is marked by unserrated, straight teeth, and external nares which are further back on the skull than in baryonychinae. And baryonychinae, which contains the genera Baryonyx and Suchomimus, is marked by serrated, slightly curved teeth, smaller size, and more teeth in the lower jaw behind the terminal rosette than in spinosaurines. Others such as Siamosaurus, may belong to either Suchomimus or Spinosaurus, but are too incompletely known to be assigned with confidence.

Phylogeny

The subfamily Spinosaurinae was named by Sereno in 1998, and defined by Holtz et al. (2004) as all taxa closer to Spinosaurus aegyptiacus than to Baryonyx walkeri. And the subfamily Baryonychinae was named by Charig & Milner in 1986. They erected both the subfamily and the family Baryonychinae for the newly discovered Baryonyx, before it was referred to the Spinosauridae. Their subfamily was defined by Holtz et al. in 2004, as the complementary clade of all taxa closer to Baryonyx walkeri than to Spinosaurus aegyptiacus. Examinations by Marcos Sales and Cesar Schultz et al. indicate that the South American spinosaurids Angaturama, Irritator, and Oxalaia were intermediate between Baronychinae and Spinosaurinae based on their craniodental features and cladistic analysis. This indicates that Baryonychinae may in fact be non-monophyletic. Their cladogram can be seen below.

Spinosauridae	Baryonyx Cristatusaurus Suchomimus Angaturama Oxalaia Spinosaurus

The next cladogram displays an analysis of Tetanurae simplified to show only Spinosauridae from Allain et al. (2012):

Spinosauridae	Spinosaurinae Irritator Spinosaurus Baryonychinae Ichthyovenator Suchomimus Baryonyx

Paleobiology

Lifestyle and hunting

Life restoration of Baryonyx with a fish in its jaws

Spinosaurid teeth resemble those of crocodiles, which are used for piercing and holding prey. Therefore, teeth with small or no serrations, such as in spinosaurids, were not good for cutting or ripping into flesh but instead to ensure a strong grip on a struggling prey animal. Spinosaur jaws were likened by Vullo et al. to those of the pike conger eel, in what they hypothesized was convergent evolution for aquatic feeding. Both kinds of animals have some teeth in the end of the upper and lower jaws that are larger than the others and an area of the upper jaw with smaller teeth, creating a gap into which the enlarged teeth of the lower jaw fit, with the full structure called a terminal rosette.

Annotated skull diagram of Spinosaurus

Spinosaurids have in the past often been considered mainly fish-eaters (piscivores), based on comparisons of their jaws with those of modern crocodilians. Rayfield and colleagues, in 2007, conducted biomechanical studies on the skull of the European spinosaurid Baryonyx, which has a long, laterally compressed skull, comparing it to gharial (long, narrow, tubular) and alligator (flat and wide) skulls. They found that the structure of baryonychine jaws converged on that of gharials, in that the two taxa showed similar response patterns to stress from simulated feeding loads, and did so with and without the presence of a (simulated) secondary palate. The gharial, exemplar of a long, narrow, and tubular snout, is a fish specialist. However, this snout anatomy doesn’t preclude other options for the spinosaurids. While the gharial is the most extreme example and a fish specialist, and Australian freshwater crocodiles (Crocodylus johnstoni), which have similarly shaped skulls to gharials, also specialize more on fish than sympatric, broad snouted crocodiles. And are opportunistic feeders which eat all manner of small aquatic prey, including insects and crustaceans. Thus, their aptly shaped snouts correlate with fish-eating, this is consistent with hypotheses of this diet for spinosaurids, in particular baryonychines, but it does not indicate that they were solely piscivorous.

Life restoration of the head of Spinosaurus

A further study by Cuff and Rayfield (2013) on the skulls of Spinosaurus and Baryonyx did not recover similarities in the skulls of Baryonyx and the gharial that the previous study did. Baryonyx had, in models where the size difference of the skulls was corrected for, greater resistance to torsion and dorsoventral bending than both Spinosaurus and the gharial, while both spinosaurids were inferior to the gharial, alligator, and slender-snouted crocodile in resisting torsion and medio-lateral bending. When the results from the modeling were not scaled according to size, then both spinosaurids performed better than all the crocodilians in resistance to bending and torsion, due to their larger size. Thus, Cuff and Rayfield suggest that the skulls are not efficiently built to deal well with relatively large, struggling prey, but that the spinosaurids may overcome prey simply by their size advantage, and not skull build Sues and colleagues studied the construction of the spinosaurid skull, and concluded that their mode of feeding was to use extremely quick, powerful strikes to seize small prey items using their jaws, whilst employing the powerful neck muscles in rapid up-and-down motion. Due to the narrow snout, vigorous side-to-side motion of the skull during prey capture is unlikely. Based the size and positions of their nostrils, Sales & Schultz (2017) suggested that Spinosaurus possessed a greater reliance on its sense of smell and a more piscivorous lifestyle than Irritator and baryonychines.

Dorsal sails

This has been compared to the dorsal fin of a sailfish. These structures have had many proposed functions over the years, such as thermoregulation, to aid in swimming, to store energy or insulate the animal, or for display purposes, such as intimidating rivals and predators, or attracting mates.

Paleoecology

Habitat

For example: Spinosaurus, from the Cenomanian of North Africa, lived in a humid, tropical environment of tidal flats and channels with mangrove forests. Similarly to the Brazilian spinosaurine Oxalaia, which also lived in the tropics, in large forests of conifers, ferns, and horsetails. Oxalaia, from the Cenomanian Alcântara Formation and, Irritator, from the Aptian-Albian Santana Formation lived in arid to semi-arid regions, experiencing short, intense rainfall followed by long dry periods. Much of the vegetation from Irritator's habitat could survive long periods without water.

From this it is apparent that Cenomanian Northern Africa and northeastern Brazil, inhabited by Spinosaurus and Oxalaia respectively, shared an extremely similar climate and many of the same biota. This is a probable result of Gondwana, a prehistoric supercontinent comprising most of the modern southern hemisphere, as South America and Africa drifted apart from each other during the Middle Jurassic, the flora and fauna on either continent would have continued to evolve separately from each other, contributing to small anatomical differences between taxa.

Spinosaurus spent much of its time in or around water.

Bone histology

A 2010 publication by Romain Amiot and colleagues found that oxygen isotope ratios of spinosaurid bones indicates semiaquatic lifestyles. Isotope ratios from teeth from Baryonyx, Irritator, Siamosaurus, and Spinosaurus were compared with isotopic compositions from contemporaneous theropods, turtles, and crocodilians. The study found that, among theropods, spinosaurid isotope ratios were closer to those of turtles and crocodilians. Siamosaurus specimens tended to have the largest difference from the ratios of other theropods, and Spinosaurus tended to have the least difference. The authors concluded that spinosaurids, like modern crocodilians and hippopotamuses, spent much of their daily lives in water. The authors also suggested that semiaquatic habits and piscivory in spinosaurids can explain how spinosaurids coexisted with other large theropods: by feeding on different prey items and living in different habitats, the different types of theropods would have been out of direct competition.

In 2018, an osteological analysis on a fragmentary tibia, (designated LPP-PV-0042) revealed osteosclerosis (high bone density) was present in the specimen, this condition had previously only been seen in Spinosaurus,

Feeding

A fossil of the fish Scheenstia, prey of Baryonyx

Direct fossil evidence shows that spinosaurids fed on fish as well as a variety of other small to medium-sized animals, including small dinosaurs. Baryonyx was found with scales of the prehistoric fish, Scheenstia, in its body cavity, and these were abraded, hypothetically by gastric juices. Bones of a young Iguanodon, also abraded, were found alongside this specimen. If these represent Baryonyx’s meal, Baryonyx was, whether in this case a hunter, or a scavenger, an eater of more diverse fare than fish. Moreover, there is a documented example of a spinosaurid having eaten a pterosaur, as spinosaurid teeth were found embedded within the fossil vertebrae of one found in the Santana Formation of Brazil. This may represent a predation event, but Buffetaut et al. consider it more likely that the spinosaurid scavenged the pterosaur carcass after its death.

Timeline of genera

External links

• Spinosauridae
• Spinosauridae on the Theropod Database